1. Field of the Invention
The present invention relates to the monitoring of the state of a foundation embedded in the ground.
Such monitoring may in fact be desirable, notably in certain situations likely to result in damage or even destruction of the foundation, and consequently of a structure borne by this foundation.
Such situations may, for example, include natural phenomena such as floods, earthquakes or landslips.
To illustrate the proposal, the non-limiting example of a bridge on piers that are partially immersed in a river is taken hereinbelow.
In certain conditions, undermining may occur at the level of the piers. This is an effect of erosion, gradual or abrupt, of the ground around and underneath the piers, caused by the flow of river water, particularly if this flow is turbulent.
Since the structure of the ground is then modified around and underneath the piers, the balance of said piers may be altered vertically and/or in rotation.
Significant undermining may lead to embrittlement or even rupture of the piers, which could cause the deck of the pier to drop suddenly.
This phenomenon definitely represents one of the main causes of bridge collapse.
It may be aggravated in the case of river flooding because, in this case, the erosion of the ground is abruptly accelerated and is accompanied by an increase in the thrust of the water on the piers and possibly impacts due to floating objects driven by the river in flood.
The rupture mode of a pier, likely to occur in such circumstances, globally follows the following sequence:                the ground is undermined gradually clearing the base of the pier;        the forces, which should be transmitted to the head of the pier and be dissipated into the ground, are applied lower down in the foundation, i.e. in an area which usually has little reinforcement and which is not engineered to take up these forces;        this effect is amplified until the foundation ruptures, followed rapidly by the rupturing of the pier as a whole.        
Such a rupture is called “brittle” rupture and is not necessarily preceded by a gradual tilting of the pier.
2. Description of Related Art
A number of techniques for detecting undermining are known.
A first group of techniques consists in taking, occasionally or periodically, a reading of the surface of the ground at the bottom of the water.
This reading may be manual, for example by using a rod from the surface, by having divers make sketches or take photographs, or by using sonars.
As a variant, the reading may be automated or semi-automated. As an example, a remotely controlled submarine equipped with a camera may be used.
Schematically, these techniques address the level of the ground on the river bed. A lower ground level indicates the existence of an undermining.
A second group of techniques consists in having permanent instrumentation to make it possible to take readings of the same type as in the preceding case, but more regularly.
The instrumentation comprises, for example, a metal collar sliding on an immersed rod inserted vertically into the ground, and a magneto-inductive measuring device for measuring the position of the collar on the rod.
In another configuration, the device may consist of a weight suspended by a cable by a toothed wheel. A measuring device measures the position of the toothed wheel, and therefore the gradual lowering of the weight.
In all cases, the measurement relies on the lowering of an object by gravity as the ground is eroded, and on measuring the position of the object. A lowering of the object also reveals a lowering of the ground level, which may reflect the existence of an undermining.
The techniques of these two groups present a certain number of drawbacks.
Since they are based on a measurement of the ground level, they allow only for the detection of the presence of an undermining formed at the position of the sensor. It is therefore possible that a foundation that is at risk will not be detected because the ground is undermined at a different point from that where the sensor is positioned, or because there is no clear undermining. The rupture of the foundation as a result of the undermining may be abrupt, as described above, so such a detection may not be early enough.
Nor are these techniques effective for use in adverse conditions, such as floods for example. That is obvious when it comes to the production of manual readings. However, even in the case of automated readings, the instrumentation placed on the surface of the water or in the water will generally not withstand the conditions.
As an example, the abovementioned rod and magnetic collar may be carried away by the current and the sonar may be damaged or even destroyed after having been struck by objects carried by the water in cases of flood.
Furthermore, while undermining is generally characterized by a lowering of the ground level, other effects capable of unbalancing the foundation until it drops may exist.
However, the abovementioned techniques are specific to the detection of undermining due to water flows and do not allow other risks that might weaken the foundation to be tracked.
Thus, a decompression of the ground, for example associated with land movements during an earth tremor, may lead to a loss of strength of the foundation (the decompacted ground no longer serves as an abutment for the foundation) without thereby significantly acting on the height of the ground. Such an effect cannot be detected with the prior art techniques outlined above.
Similarly, nor will a build-up of sediments, gradual or abrupt as a result, for example, of a landslip, be detected by the automated monitoring systems that rely on the heavy weight principle.
Moreover, the correct operation of the abovementioned techniques is difficult to control remotely. It is not possible, for example, to know if the measuring system based on a weight placed on a rod or suspended on a wire is jammed, by an object carried by the river for example, or because the components are corroded thereby. If the weight is jammed, the undermining will not be detected, and there will be no way of knowing about it without carrying out an in-situ check.
Nor do these techniques allow for the effectiveness of a repair to be checked. If an undermining is detected, it will be backfilled, generally with rubble. The prior art techniques do not make it possible to assess the capacity of this repair to provide the foundation with the necessary horizontal abutment.
An article by Y. Fujino and D. M. Siringoringo entitled “Structural health monitoring for risk assessment of bridges: concept and implementations” and published in November 2008 very briefly raises the possibility of providing piers with a bridge of inclinometers, in order to detect their collapse as a result of an undermining.
However, the inclination of a bridge pier may be normal, in particular when it occurs in response to a strong horizontal thrust exerted by the river water. In itself, it therefore does not constitute a relevant indicator.
Furthermore, given that the rupture of the pier may be abrupt as described above, it is in fact the collapse of the pier that is observed by this technique. The technique cannot, in practice, anticipate the collapse.